In marine seismic it is common to carry out seismic surveys in which several long streamers and sources of seismic signals, usually airguns, are towed behind a vessel. The seismic signals, usually in the form of shock waves from the airguns, are reflected from layers in the underground and picked up by the streamers, after which the signals are interpreted, giving information on the geology of the underground. The streamers must be held apart so that they lie side by side in the longitudinal direction and have a certain spacing in the width direction. This is usually done with wing-shaped hydrofoils called deflectors or paravanes. Each deflector is usually attached to a towing block, called a “bridle block” in the trade, by straps at the forward and aft ends of the deflector. In the trade, the straps are referred to as “bridle lines” or “door bridles”. Usually two or more bridle lines extend from the forward edge of the deflector to the bridle block, and two or more bridle lines from the aft edge of the deflector to the bridle block. The bridle block, in turn, is attached to the towline from the towing vessel. As a rule, the deflectors are placed on either side of the seismic tow and have such an angle of attack in the water that they pull the towlines and thereby the tow laterally outwards.
There is often a need to change the pull of the deflectors on the tow, and to control the direction of the tow. This may be done by adjusting the angle of attack of the deflector in the water. The angle of attack can be adjusted by replacing the bridle lines at the forward or aft edge of the deflector with shorter or longer bridle lines, while the rest of the bridle lines are kept unchanged. Another way of adjusting the angle of attack is to replace spacers, which include attachment points for the bridle lines in the bridle block, with shorter or longer spacers, whereas the bridle lines are typically kept in the same lengths.
The patent publication GB 2399883 relates to a deflector in which the angle of inclination may be varied to control how deep the deflector will move. A bridle block may be provided with an actuator or a hydraulic cylinder to change the angle of inclination of the deflector.
The Norwegian patent 331725 relates to a bridle block for a deflector including a main part or stem and a rocker arm which are pivotably attached to each other on a pivot axis, the stem having an attachment for a towline from a vessel, a forward portion of the rocker arm having an attachment for fore bridle lines of the deflector, and an aft portion of the rocker arm having an attachment for aft bridle lines of the deflector. A torque arm is connected to the forward portion of the rocker arm to apply a torque to the rocker arm. The torque arm is connected to a length-adjustable so-called “seventh bridle line” extending to the aft edge of the deflector, for the control of the deflector. The length-adjustment of the “seventh bridle line” is carried out with a linear actuator placed on the deflector. The linear actuator may be a hydraulic cylinder or winch. The linear actuator may be remote-controlled from a vessel towing the bridle block.
On a failure of the remote control of a deflector, for example on a power failure, it is desirable that the devices adjusting the bridle lines take a failsafe position in which the deflector is controlled into an angle of attack at which its pull on the tow is acceptable.